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Allow shifting macro arguments by a negative amount

Fixes #733

--- a/src/asm/macro.c

+++ b/src/asm/macro.c

@@ -168,12 +168,12 @@

 {

 	if (!macroArgs) {

 		error("Cannot shift macro arguments outside of a macro\n");

-	} else if (count < 0) {

-		error("Cannot shift arguments by negative amount %" PRId32 "\n", count);

 	} else if (macroArgs->shift < macroArgs->nbArgs) {

 		macroArgs->shift += count;

 		if (macroArgs->shift > macroArgs->nbArgs)

 			macroArgs->shift = macroArgs->nbArgs;

+		else if (macroArgs->shift < 0)

+			macroArgs->shift = 0;

 	}

 }

--- a/src/asm/rgbasm.5

+++ b/src/asm/rgbasm.5

@@ -1487,6 +1487,7 @@

 .Pp

 .Ic SHIFT

 can optionally be given an integer parameter, and will apply the above shifting that number of times.

+A negative parameter will shift the arguments in reverse.

 .Ss Printing things during assembly

 The

 .Ic PRINT

--- a/test/asm/shift-negative.asm

+++ b/test/asm/shift-negative.asm

@@ -1,4 +1,16 @@

+reverse: MACRO

+	for i, _NARG

+i = _NARG - i - 1

+		shift i

+		println \1

+		shift -i

+	endr

+ENDM

+

+	reverse $1, $2, $3

+

 m: MACRO

+	shift 2

 	shift -3

 ENDM

 	m

--- a/test/asm/shift-negative.err

+++ b/test/asm/shift-negative.err

@@ -1,3 +1,0 @@

-ERROR: shift-negative.asm(4) -> shift-negative.asm::m(2):

-    Cannot shift arguments by negative amount -3

-error: Assembly aborted (1 errors)!

--- a/test/asm/shift-negative.out

+++ b/test/asm/shift-negative.out

@@ -1,0 +1,3 @@

+$3

+$2

+$1